1. Field of the Invention
This invention relates to an optical writing head, and more particularly to an optical writing head for use with a printer or like machines wherein an electro-photographic process is employed.
2. Description of the Prior Art
A conventional optical writing head is first described with reference to FIG. 12 which schematically illustrates general construction of an LED (light emitting diode) printer in which an LED array is employed for such an optical writing head.
Referring to FIG. 12, a photosensitive drum 201 having a surface layer made of a photoconductive material rotates in the direction indicated by an arrow mark. The drum 201 is at first charged uniformly by a charger 202 and then exposed to light in accordance with information of an image to be recorded by an LED array head 203 so that an electrostatic latent image is formed on the photoconductive layer of the drum 201. The latent image is then developed by a developer 204 to form a toner image which is subsequently transferred to transfer paper 206 under action of a transfer device 205. The transfer paper 206 to which the toner image has been transferred is then fed to a fixing device 207 at which the toner image is fixed to the transfer paper 206. Meanwhile, the photosensitive drum 201 from which the toner image has been transferred is then cleaned by a cleaner 208 so that it may be used again.
The LED array head 203 includes an LED element array consisting of a large number of LED chips arranged in a row in the direction of a print column as hereinafter described, and a self-condensing type rod lens array for focusing light emitted from the LED elements of the LED element array on the photosensitive drum 201.
Referring to FIG. 13, the conventional LED array head 203 includes an LED chip 222 die bonded on a ceramic substrate 221, a bonding wire wire-bonded to each of light emitting portions of the LED chip 222 for energization of the light emitting portions and connected to the LED chip 222, and a glass cover 226 for protecting a conductor pattern on the ceramic substrate 221 on which the LED chip 222 is mounted. The bonding wires and the glass cover 226 are mounted on a heat radiating plate 223 made of aluminum or a like material, and a mounting member 225 for a self-condensing type rod lens array 224 is supported on the heat radiating plate 223.
In such an LED array head as described just above, when LED chips are to be mounted on a ceramic substrate, it is necessary to hold positioning tolerances in linearity and so on as close as .+-.10 microns or so. However, since the width of a pad for tie bonding is 1.2 to 1.5 mm or so and hence is considerably great comparing with the width of a chip which is 0.7 to 1 mm or so, positioning of such a chip requires an optical technique which adopts a TV camera or the like. Accordingly, the efficiency in die bonding of LED chips is low, and an expensive mounting apparatus must necessarily be used.
Meanwhile, wire bonding must be done once for each light emitting portion of an LED chip (up to 64 light emitting portions are normally formed for one LED chip). For example, in the case of a commonly used LED array head for A4 letter size printing with an integration density of 300 DPI (dots per inch=about 12 dots/mm), up to 2,560 bonding wires are necessitated, and it takes about 40 minutes to one hour or so to complete such wire bonding even if a high speed wire bonder is used. Besides, even if one of the 2,560 bonding wires is bonded incompletely, the entire LED array head will be a rejected article. Accordingly, such wire bonding as described above requires close attention, which deteriorates the production efficiency and raises the production cost.
Further, since variations in tolerance in alignment of LED chips of an LED array head and in linearity in the direction of height of light emitting portions must necessarily be within severe tolerances of .+-.100 microns over the entire width (216 mm in the case of the A4 size) of the LED chips, the warp and bend of a ceramic substrate itself are restricted. Accordingly, a ceramic substrate itself must necessarily be high in accuracy and 100% inspection is required, which makes substrates expensive.
Further, since a self-condensing type rod lens array is carried on a heat radiating plate on which a ceramic substrate is mounted, errors in dimension of those elements are accumulated. Accordingly, it is also a drawback that a required degree of accuracy is not attained. Tolerances in positioning of an existing rod lens array of the self-condensing type are .+-.0.1 mm over the entire length of the array, which requires optical confirmation of tolerances in mounted position of the array and requires provision of the LED array head with a mechanism for enabling positioning with a high degree of accuracy. Accordingly, the production cost is raised in regard to man-hours for assembly, production equipment, number of parts and so on.